The present invention relates to a hydraulic control system for an automatic transmission.
One of prior art hydraulic control system for an automatic transmission is disclosed, for example, in JP-A 2-178466. Referring to FIG. 12 which shows only a part relative to shift between second, third, and fourth speeds, this hydraulic circuit is used in the same power transmission mechanism as that one which will be described later in connection with preferred embodiments of the present invention. This power transmission mechanism is for the use of four forward speeds, and has a brake which is engaged at second speed, released at third speed, and engaged at fourth speed. Operation of this brake is controlled by a hydraulic servo apparatus which has a second speed apply chamber 42, a third speed release chamber 46, and a fourth speed apply chamber 52. When supplying hydraulic pressure, the second speed apply chamber 42 makes a band brake engaged. When supplying hydraulic pressure, the third speed release chamber 46 makes the band brake released even if the second speed apply chamber 42 undergoes hydraulic pressure. When supplying hydraulic pressure, the fourth speed apply chamber 52 makes the band brake engaged regardless of the state of hydraulic pressure within the second speed apply chamber 42 and the third speed release chamber 46. The hydraulic circuit as shown in FIG. 12 includes a shift valve 100, a shift valve 102, a 4-2 sequence valve 104, and a 4-2 relay valve 106. The shift valves 100, 102 are switched by signal pressure out of solenoids 108, 110, respectively. Upon second speed, the shift valve 100 is positioned on the side that a hydraulic passage 112 receiving hydraulic pressure in D-range communicates with the second speed apply chamber 42, whereas the shift valve 102 is positioned on the side that hydraulic passage 114 communicating with the third speed release chamber 46 is drained. Upon third speed, the shift valve 100 occupies the same position as that one upon second speed, whereas the shift valve 102 is positioned on the side that the hydraulic passage 112 receiving hydraulic pressure in D-range communicates with the third speed release chamber 46. Upon fourth speed, the shift valve 102 occupies the same position as that one upon third speed, whereas the shift valve 100 is positioned on the side that a hydraulic passage 114 communicating with the third speed release chamber 46 is connected to a hydraulic passage 116 communicating with the fourth speed apply chamber 52.
The 4-2 sequence valve 104 and the 4-2 relay valve 106 are arranged to carry out 4-2 shift and 3-2 shift at a predetermined timing. That is, when shifting from fourth speed to second speed, both shift valves 100, 102 are switched at the same time. In this case, since the band brake is in the engagement state both at second speed and at fourth speed, it is necessary to avoid the band brake being released during shift in view of measures against shift shock. Thus, the 4-2 sequence valve 104 is constructed to be switched by hydraulic pressure within the hydraulic passage 114, i.e., hydraulic pressure within the third speed release chamber 46, and it functions so that until this hydraulic pressure reduces, hydraulic pressure within the fourth speed apply chamber 52 fails to decrease. This allows hydraulic pressure within the third speed release chamber 46 to be discharged with the engagement state of the band brake maintained, carrying out 4-2 shift. In addition to the 4-2 sequence valve 104, the 4-2 relay valve 106 is arranged for smooth operation of 3-2 shift. That is, upon 3-2 shift, the shift valve 102 is switched to thereby discharge hydraulic pressure within the third speed release chamber 46. At this moment, since the fourth speed apply chamber 52 is connected to the hydraulic passage 112 through the shift valve 100, the 4-2 sequence valve 104, and the shift valve 102, the fourth speed apply chamber 52 temporarily undergoes hydraulic pressure, resulting in possible shift of third speed.fwdarw.fourth speed.fwdarw.second speed. That is, since the fourth speed apply chamber 52 is supplied with hydraulic pressure until hydraulic pressure within the third speed release chamber 46 decreases, it is necessary to cut it off upon 3-2 shift. This is carried out by the 4-2 relay valve 106.
However, such known hydraulic control system for an automatic transmission is constructed so that the 4-2 sequence valve is arranged for appropriate operation of 4-2 shift, and the 4-2 relay valve is arranged to prevent inconveniences from occurring upon 3-2 shift, necessitating two valves in addition to the shift valves.
It is, therefore, an object of the present invention to provide a hydraulic control system for an automatic transmission which allows smooth shift without any valves for timing adjustment.